1. Field of the Invention
The present invention relates to a method and apparatus for the treatment of water containing silica. More particularly, the invention relates to a method and apparatus to treat water utilizing a novel combination of pretreatment and reverse osmosis (RO) processes that result in a high percentage of treated water output while lowering the amount of chemical additions used in the process and lowering the amount of process waste.
2. Background of the Invention
Increasing demand for fresh water supplies has led to an increasing need to utilize brackish water sources. At the same time, industries extracting inland groundwater must meet strict limitations prior to discharging that water.
These brackish waters usually contain silica at concentrations that limit to the recovery of treated water for use or discharge. When these brackish waters are used as a water supply, it is beneficial to provide a treatment method that recovers as much treated water as possible. When brackish water sources are proposed for use or discharged as a by-product of an industrial process, the cost of disposal of the concentrated waste stream is a primary consideration, as the purification technologies employed all generate a waste stream. Maximizing the treated water and minimizing the waste stream are common goals of many treatment methods.
Depending on the treatment technology employed, the treated water quality desired and the location of the source and options for waste stream disposal, treatment methods such as ultra-violet (UV) oxidation, micron filtration (MF), ion exchange (IX), reverse osmosis (RO), evaporation and some combinations thereof have been employed.
One issue related to addressing current solutions to this problem is the volume of the waste stream being generated by the conventional treatment methods. Because the current processes are volumetrically inefficient, a significant waste stream is usually generated. Various disposal options for the waste stream exist. Disposal options are generally limited to (a) discharges to surface waters, (b) deep-well disposal, (c) discharges to existing wastewater treatment plants, or (d) natural or mechanical evaporation. All of these disposal options are either not feasible or are costly for reasons specific to that option. In (a) a suitable receiving water must be nearby and environmental regulations must be met. In (b) the availability and costs of the wells and transmission lines to a suitable well site meeting environmental regulations must be considered. In (c) existing wastewater facilities must be nearby and may require capacity increases or unit process modifications to handle the flows. In (d) the costs of evaporation can increase the cost of treated water significantly. Currently, individual treatment technologies are typically applied and the resulting processes either do not operate at high enough efficiency, or still result in a significant amount of concentrated liquid waste stream that must be disposed of.
As increased efficiencies have been pursued, solutions are available that create additional issues. For example, it is well understood that a major obstacle for increased water recovery utilizing RO solutions is the presence of silica in the water that creates scaling problems as its concentration increases through more efficient recovery processes. One solution meant to address the silica problem to date involves continuously using acid and/or caustic additions to adjust the natural pH of the process stream to reduce the risk of scaling and allow higher water recoveries. One such solution is disclosed by U.S. Pat. No. 5,925,255, Mukhopadhyay, filed Aug. 12, 1997, which is herein incorporated by reference in its entirety. The required addition of chemicals to the water treatment process increases the cost and complexity of this particular solution.
Solutions such as adding antiscalants to prevent precipitation of hardness ions and/or silica have also helped increase the recovery rate of some treatment processes. However, it has not been the general practice in the industry to operate RO systems on brackish water sources at greater than 90% recovery even with the addition of acid or an antiscalant/sequestriant because of the scaling potential risks, thus limiting the water sources that can be recovered.
Because of these issues and other reasons, the commercial viability of extremely high RO system recoveries where the concentration of solutes in the final reject is 10 to 100 times the water source stream therefore has not been established in practice.
Because the RO process concentrates and separates simultaneously, the common teachings in the art are to adjust pH of the feed stream to an RO system so that the ions, when concentrated by the process, remain in solution and prevent scaling of the membrane. The teaching is also that the water recovery of the RO system is limited by the ion solubility product (Ksp) of the sparingly soluble ion pairs in the system, and that reducing water recovery is a mitigating design response if pH adjustment is not sufficient. Given these challenges, chemical adjustment of the feed stream has been taught as the least expensive way to keep feed stream constituents in solution.
Consistent with these approaches, popular water treatment system modeling programs, such as those offered by Dow and Hydranautics, put an upper limit of 1.5 times silica solubility as the upper limit of an aggressive water treatment method. At 25° C., this represents a silica concentration of somewhere around 120-130 mg/L. In systems where silica is considered to be more than 1.5 times the solubility, lowering RO system recovery is recommended by the major membrane manufactures.
As water is becoming increasingly expensive, in short supply, or both, it is desirable to further increase the ratio of the recovered water stream to the water source stream treated by RO systems.
Therefore, there exists a need for a cost-effective water treatment process that operates at the highest efficiency of water recovery possible, maximizing the efficiencies of several processes to minimize the process waste stream volume and maximizes the treated water stream.
There also exists a need for a water treatment method that minimizes the scaling caused by the presence of silica in a water treatment process, which is often the limiting factor for high water recovery levels.